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M. Grimm et al. / Geomorphology 00 (1995) ()()()...()()(J 7 Table 2 Estimated paleoflood discharges and largest gaged discharges for the four primary study sites Site (elev., m) psr Paleo-Q (mJ S-I) Gaged Q (m' s") Unitae(m3s-1 Stream power (WI Critical stream and date Ion-') m') power (W/m2)t Turkey Creek FB~lb 74 77 (1969) 0.57 1370 1365 ( 1768) FB-2 74 0.57 Lower Bear Creek FB-! 113c 117 (1983); 0.27 5205 2026 (1795) FB-2 (estimated to be 131 (1934); inundated annually) FB-2 176 (1938); 1W I:Ili (1933); 244 (1896) Cub Creek (2255) SWD' !7 7 (1980) 0.13 207 196 Upper Bear Cr. FB-! 17 13 (1980) 0.Q7 69 148 (2268) .PSI: Paleostage indicator. "Fa: Rood boulder bar. cPreferred discharge resulting from sensitivity analysis. dSWO: Slackwarer deposit. ~atio based on paleodischarge and equal to estimated peak: paleodischarge divided by drainage area rCriticaI stream power per unit area estimated to be necessary to entrain the Dmu. after Costa ( 1983). Comparisons of stream power per unit area in the channel thalweg at peak discharge, and critical stream power using Costa's relations developed for Front Range channels (Costa, 1983), indicates that critical stream power is exceeded at the two lower-elevation sites, but not at the highest elevation site (Table 2). Actual stream power is relatively low at the Turkey Creek site as a result of a relatively wide channel (45 m, as compared to 18 m at the other sites) and low gradient. Critical stream power is barely exceeded at the Cub Creek site, where a steep, confined channel produces high values of stream power. We consider these calculations to provide order-of-magnitude esti- mates because, as noted by Costa (1983), there are potentially large uncertainties involved in these calcu- lations, A sensitivity analysis conducted at the Lower Bear Creek site evaluated the effect of channel fill or scour, channel gradient, and roughness coefficients on the water-surface profile. Estimated ranges for these vari- ables were based on data from recent floods in the Colorado Rockies (McCain et aI., 1979; Jarrett and Costa, 1986; Jarrett, 1987, 1990). The hydraulicfactors and computed discharges are listed in Table 3. Results of the sensitivity analysis indicate that two factors account for the greatest variability in the dis- charge estimates. These factors are uncertain flow- resistance coefficients and the water-surface elevation of the paleofloods. Changes in channel configuration since the flood (that is, the area of the flood bar in cross sections) did not substantially affect discharge esti- mates at this site because the bar is located on the inside of a channel bend in an area of relatively ineffective flow. In addition, the area of the bar is relatively small compared to the total cross-sectional area. We believe that the sensitivity analysis for the Lower Bear Creek site is representative of the magnitude of uncertainty in discharge estimation for each of the four study sites. Therefore, we believe that the trend of decreasing unit discharge with increasing elevation (Table 2) is accu- rate. 4.2. Geochronology Results of the radiocarbon and dendrochronologic analyses are summarized in Table 4. These results very loosely bracket the ages of the flood deposits. We believe the radiocarbon ages to be more representative of the age of the flood deposits than the dendrochron- Journal: GEOMOR Aniele: 368